The correct answer is B.
In stories, there are two types of conflicts: internal and external. Internal conflicts are the mental, spiritual or emotional struggles of a character.
An external conflict is a struggle a character faces against an outside force.
The are four main types of external conflict: character vs. character, character vs. society, character vs. nature and character vs. technology.
In the external conflict, the main character struggles against an animal, the weather or a terrain and he is most often motivated by simple survival.
In the above example, both characters struggle against nature ( the fallen tree) and their conflict is external.